Spray guns for the electrostatic deposition of powder coating material generally includes a housing having a barrel through which a gas-borne or fluidized stream of the solid particulate coating material flows to an attached nozzle or spray cap. The material is electrostatically charged as it passes through the nozzle and is subsequently directed toward a workpiece which is to be coated with the material. The material is charged by means of an electrode which is connected to a high voltage power supply. In a preferred design, the charging electrode is in the form of a needle located in the center of the material flow path through the nozzle. When a high voltage is applied between the workpiece and the needle electrode, the resulting electrostatic field around the electrode charges the material as it flows through the nozzle. A diffuser assembly, which supports the needle electrode, is clamped between the barrel and the nozzle, which screws onto the barrel.
It is generally desirable to have an electrically insulating nozzle extend past the electrode to prevent an accidental shorting of the electrode to the workpiece or to any other grounded object and to reduce the chance of electrical shock for the spray gun operator. It is also desirable to removably attach the nozzle to the spray gun barrel to facilitate maintenance such as cleaning the nozzle. Typically, the nozzle is threaded onto the barrel. However, there is a danger that the nozzle will loosen during use, particularly when the spray gun operator is careless in tightening the nozzle. If the nozzle should loosen beyond a predetermined point, a potential spark path will be produced from some portion of the gun charging circuit to some electrically grounded object. This spark path is dangerous to the operator and also increases the risk of a spark which is particularly hazardous since many fluidized powdered coating materials are potentially explosive.